Use src/lib/w32/w32sockets.c for client

Move client's w32_connect() to w32sockets.c.

Replace w32_recv() and w32_send() by read() and write().

Replace w32_close() by w32_close_function.

Replace call of WSAStartup() in w32_sysdep_init() by
w32_socket_init().

Remove the identical copies of fd_is_socket(),
w32_set_winsock_errno(), w32_socket().

src/client/main.c

@@ -39,8 +39,8 @@
 #include <stdlib.h>
 #ifndef _WIN32
 #include <pwd.h>
-#include <unistd.h>
 #endif
+#include <unistd.h>
 #include "misc.h"
 #include "version.h"